The present invention relates to electrolytes which are useful for various electrochemical elements. More particularly, there is provided a mixture of quaternary ammonium salts which are useful at low temperatures, especially for use in double layer capacitors in combination with a nitrile solvent.
The basic components of electrical capacitors include conductive electrodes connected to an electric power supply and a dielectric material separating the electrodes. Electrolytic capacitors and electrochemical double layer capacitors also have an electrolyte. In an electrolytic capacitor, the dielectric is provided by an oxide layer formed on a metal foil and the electrolyte provides electrical contact to the opposite electrode. The inherently high resistance of electrolytic capacitors is generally mitigated by rolling a large sheet of the material into a roll. In an electrochemical double layer capacitor, the dielectric is provided by the electrolytic. In this type of capacitor, the resistance of the electrolyte is a significant factor in the total device resistance. In capacitors that use electrolytes, the electrolyte also has a major influence on the temperature performance of the capacitor.
Electrochemical double layer capacitors, including supercapacitors, typically comprise electrodes, electrical contacts to a power supply, separators for electrode and/or cells, an electrolyte and environmental seals. As mentioned above, a key component of electrolytic and electrochemical double layer capacitors is the electrolyte, which typically comprises a combination of a salt and a solvent. Desirable electrolytes are typically liquid with low viscosity, low density, and high conductivity over a range of ambient temperature conditions. They should also be commercially inexpensive, chemically and electrochemically stable, and compatible with carbon. Aqueous electrolyte systems have been used extensively, but certain organic liquid systems are less prone to form gas and can be more effective in providing higher energy densities over a wider usable range of temperature and potential. In addition, these organic electrolytes permit higher voltage and therefore results higher capacity in the capacitors. A need exists for improved electrolyte systems that provide optimum capacitance for capacitors to achieve high power density, a wide temperature range, and a long lifetime without memory effects.
The key requirements for both non-aqueous batteries and capacitors are low temperature performance, electrochemical stability, and lower costs.
European patent application EPO 0984471 A2 which is herein incorporated by reference, discloses a method for producing electric double layer capacitors which contains tetrafluoroborate and hexafluorophosphate of imadazolium derivatives, which can be used in the present invention. However, acetontrile or similar solvents are not utilized.
U.S. Pat. No. 5,418,682 to Warren et al, which is herein incorporated by reference discloses a method of preparing tetraalkyl ammonium tetrafluoroborate salts for use as electrolytes with dinitrile mixtures as solvents.
U.S. Pat. No. 5,965,054 to McEwen et al, which is herein incorporated by reference discloses non-aqueous electrolytes for electrical storage devices utilizing salts consisting of alkyl substituted, cyclic delocalized aromatic cations and their perfluoro derivatives with alkyl carbonate solvents.
Of the quaternary ammonium tetrafluoroborate salts, it has been found that tetraethylammonium tetrafluoroborate (TEATFB) is the most advantageous because of its stability and conductivity. However, a major disadvantage is that its solubility is limited and it starts to freeze out at about xe2x88x9230xc2x0 C. from solutions, thereby lowering low temperature performance.
The present invention comprises an improvement in non-aqueous electrolytes for use in non-aqueous batteries and double layer capacitors as the result of using a mixture of conductive quaternary ammonium tetrafluoroborate salts in a nitrile solvent. More particularly there is provided a composition which at a concentration of about 0.6 to 2.5M has a conductivity in the range of about 50 to 58 mS/cm at 20xc2x0 C. and can be cooled to at least 40xc2x0 C.
It has been found to be advantageous that one of the quaternaly ammonium tetrafluoroborate salts is tetraethylammonium tetrafluoroborate, or methyltriethylammonium tetrafluoroborate also, the use of acetonitrile as the solvent.
It is therefore an object of the invention to provide a non-aqueous electrolyte comprising mixtures of conductive quaternary ammonium tetrafluoroborate salts which can be used in batteries and double layer capcitors.
It is a further object of the invention to provide a non-aqueous electrolyte capable of performance at low temperatures.
It is a still further object of the invention to provide an improved double layer capacitor.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims, taken in conjunction with the following drawing.